Sodium Insertion into Fe[Fe(CN)6] Framework Prepared by Microwave-Assisted Co-Precipitation

Prussian blue analogues (PBAs) are interesting materials for Na electrochemical insertion, owing to their ease of synthesis, rigid open framework and large interstitial space. In this work, iron hexacyanoferrate nanoparticles, FeIII[FeIII(CN)6], also known as Berlin green (BG), where both Fe centers are in the Fe(III) configuration, have been synthesized by an easy microwave assisted co-precipitation method, with an average particle size of around 44 nm. The BG nanoparticles exhibited a 120 mAh g−1 specific capacity with a capacity retention of 84.8 % over 100 cycles at 20 mA g−1 current and up to 80 mAh g−1 at 10 C rate. The initial charge and discharge processes were fully investigated by ex-situ X-ray absorption spectroscopy (XAS). The XAS results reveal that there is some irreversibility in the initial electrochemical processes and the Fe−C and Fe−N bond lengths change during the first charge/discharge cycle. This effect was attributed to BG electrode activation processes, and partly explained the observed capacity fading.